Share flash between Rails 2 and Rails 4 with monkey patches! Works both ways with sweeping! If you actually have this problem, *facepalm*

rails_2_rails_4_flash_sharing.rb

# Rails 2.2.2.2
# vendor/bundle/ruby/1.8/gems/actionpack-2.2.2.2/lib/action_controller/flash.rb

        # Access the contents of the flash. Use <tt>flash["notice"]</tt> to read a notice you put there or 
        # <tt>flash["notice"] = "hello"</tt> to put a new one.
        # Note that if sessions are disabled only flash.now will work.
        def flash(refresh = false) #:doc:
          if !defined?(@_flash) || refresh
            @_flash =
              if session.is_a?(Hash)
                # don't put flash in session if disabled
                FlashHash.new
              else
                # otherwise, session is a CGI::Session or a TestSession
                # so make sure it gets retrieved from/saved to session storage after request processing
                session["flash"] ||= FlashHash.new # <-- will use Rails 4 hash if exists, but doesnt know it

                # leaving these lines in as they orchestrated the patch
                # Rails.logger.info "!!!! #{session["flash"].class}"
                # Rails.logger.info "!!!! #{session["flash"].inspect}"

                if session["flash"].class == Hash
                  # convert Rails 4 Hash to Rails 2 FlashHash
                  new_hash = FlashHash.new
                  session["flash"]["flashes"].each do |k, v|
                    new_hash[k] = v
                  end
                  session["flash"] = new_hash
                end

                session["flash"] # actually return the converted object

              end
          end

          @_flash
        end
        
        
  # Rails 4.0.2
  # vendor/bundle/ruby/1.9.1/gems/actionpack-4.0.2/lib/action_dispatch/middleware/flash.rb
  # Rails 2 just uses a hash under session['flash'] whereas Rails 4 uses a more complicated structure
  # after sweep, this comes back as an empty hash to Rails 4.. which then breaks flash completely
  # only required thing to do is restore session['flash']['flashes'] .. discard can and will default to [] which is fine
  # note: this doesn't end up using the Rails 3 code because.. well it's Rails 3 and Rails 2 used FlashHash internally
  # but doesnt seem to serialize it into the session, fooling Rails 4 into thinking it's a Rails 4 style flash Hash
    def flash
      # Rails 2 stores Flashes different
      session["flash"]["flashes"] ||= {}
      # move Rails 2 flashes to Rails 4 flashes location
      session["flash"].each do |k, v|
        if k.class == Symbol # hopefully skip "flashes", "discard"
          session["flash"]["flashes"][k] = v
          session["flash"].delete(k)
        end
      end
      @env[Flash::KEY] ||= Flash::FlashHash.from_session_value(session["flash"])
    end

related errors:

undefined method `keys' for nil:NilClass